1. Field of the Invention
The present invention relates to a scale holding device and an encoder.
2. Description of the Related Art
As devices for measuring positions and angles, there are optical-type encoders which include a scale serving as a measurement reference, a light projecting unit for projecting light onto the scale, and a light receiving unit for receiving light reflected from the scale. Such encoders include a linear encoder including an elongated (straight edge) scale, typically in the form of a metallic tape-like scale, and there are some methods (configurations) for fixing the scale to a mounting object. For example, the first method is a method for directly affixing a tape-like scale to a mounting object using a double-sided tape or the like. In this method, a highly-adhesive double-sided tape is typically used such that the scale does not peel off from a mounting object, resulting in a difficulty in repositioning (reusing) the scale. Thus, an accurate technique is required, to avoid the need for repositioning of the scale, upon affixing the scale to a mounting object. In addition, a dedicated tool may also be needed such that the scale is mounted at a desired position. Furthermore, when the scale is fixed with only double-sided tape, positional deviation may occur between a mounting object and the scale due to the deterioration of the adherence of double-sided tape over-time. The occurrence of positional deviation is difficult to recognize by direct measurement, resulting in an adverse effect on the measurement accuracy of the encoder. Furthermore, the second method is a method for fixing a tape-like scale to a mounting object via a dedicated holding member (attachment plate). Japanese Patent No. 4477440 discloses an elastic scale holding method for adhesively fixing a scale to a holding member (scale fixture) having elasticity or non-elasticity for different purposes. According to Japanese Patent No. 4477440, the effect of thermal expansion caused by temperature change is reduced and a portion of the adhesive is made to be a non-elastic layer which is used as a reference at the time of extension/contraction.
However, the holding method disclosed in Japanese Patent No. 4477440 is only suitable for a glass scale or the like formed of a hard material but is not suitable for a tape-like scale or the like having flexibility. A specific description will be given under the assumption that, when such a holding method is employed, a scale is firstly adhesively fixed to a holding member and then the holding member is fixed to a mounting object. Also, a reference member (non-elastic layer) for performing positioning of the scale with respect to the holding member by firmly fixing the scale to the mounting object is present at one portion (cross-section from one lateral side to the other lateral side) of the rear side of the scale. Hence, if the surface of the mounting object is distorted, such distortion is directly transmitted to the scale when the holding member with the scale being held thereon is mounted on the mounting object. Thus, when a tape-like soft or flexible scale is used, it is highly probable to exert an adverse effect on the measurement accuracy of the encoder. Furthermore, in the holding method disclosed in Japanese Patent No. 4477440 in connection with the first method, the rear side and the lateral sides of the scale are adhesively fixed to the holding member via an elastic layer in addition to the reference member, and thus, repositioning cannot be achieved when a tape-like scale is used. In particular, when the tape-like scale is affixed to the holding member, it is difficult to perform unique positioning of the scale with respect to the holding member, and thus, the scale cannot be repositioned, which is a disadvantage in terms of the efficiency (ease) of mounting operations.